Wan robber upset by animal (5,4)
I believe the answer is:
brown bear
'animal' is the definition.
(thesaurus)
'wan robber upset' is the wordplay.
'upset' indicates an anagram.
'wan'+'robber'='wanrobber'
'wanrobber' is an anagram of 'BROWN BEAR'.
'by' is the link.
(Other definitions for brown bear that I've seen before include "Fierce animal" , "beast" , "animal of North America" , "North American ursine animal" , "Large mammal" .)